The present invention generally relates to a method for monitoring the etching condition of a chemical etching process, and more specifically to a real-time in-situ method for measuring the width of bottom under cut.
Film etching is an important micro lithographic technique that uses a photoresist pattern as a mask for film patterning in semiconductor device fabrication. In general, etching techniques can be categorized as wet etching and dry etching. Wet etching relies on acid or alkali etching solution to etch away film material. Because an etching solution also etches away the film material from the two edges under the photoresist pattern, wet etching is isotropic. The width of the etched pattern on the bottom is smaller than that of the pattern designed with photoresist. The phenomenon of bottom under cut is very pronounced in wet etching.
Dry etching usually involves plasmas and is anisotropic. A plasma can be considered as an ionized gas consisting of positive and negative ions, electrons, neutrals, atoms, molecules, clusters, etc. Plasma etching relies on the fact that with the right chemistry, surface atoms can be turned into gas phase molecules which can then be pumped away. A glow discharge plasma (usually RF) is used to generate reactive species, e.g. atoms, radicals and ions, from parent gases for plasma etching.
Anisotropic etching can be achieved by means of appropriate etching condition and vertically injected ions. The size and dimension of the etched patterns can be close to the photoresist mask patterns. In practice, however, different materials may be etched away near the edge of the photoresist patterns by the neutralized particles. In addition, the ions in the plasma may be scattered by the grid electrodes while they are directed towards the wafer substrate. The charge on the substrate surface also changes the incident angles of the ions. Therefore, the side walls of the etched pattern also react with the ions to result in bottom under cut.
In order to fabricate semiconductor devices having fine patterns and features in the sub-micron range, different methods and apparatus have been presented to improve the fabrication process and to control the bottom under cut in the etched patterns. A popular technique used in developing new process technology is to vary the parameters of plasma etching. From the etching results such as etched rate and shape, the effect of varying the parameters can be estimated and determined. The information is then fed back to the parameters to iteratively achieve the desired process. U.S. Pat. No. 5,573,624 and U.S. Pat. No. 5,788,801 granted to Barbee et al. that disclose a contactless method and apparatus for real-time in-situ monitoring of a chemical etching process during etching are examples in the art.
The conventional technique by varying parameters used for plasma etching can not determine how much the line width or critical dimension has been different from the designed or expected value. In addition, it is not possible to monitor and determine the bottom under cut in real time invasively with current technology of critical dimension measurement when bottom under cut is necessary. There exists a strong demand in having an invasive and real time technology that can monitor and measure the width of bottom under cut during the etching process of semiconductor device manufacturing.
This invention has been made to overcome the above-mentioned deficiency in determining the bottom under cut in the conventional technology. The primary object of the invention is to provide a method for measuring the width of bottom under cut during the etching process while manufacturing the semiconductor device. Another object is to provide a method for measuring the width without having to damage the manufactured semiconductor device.
Accordingly, an oxide line pattern is first formed on a substrate and the width of the line pattern is measured. A deposition layer is then deposited above the oxide line pattern and the substrate. A photoresist pattern is formed by a micro-lithographic technique above the deposition layer. The width of the photoresist pattern is measured. The deposition layer is then etched to form a deposition pattern by using the photoresist pattern as a mask.
The residual deposition layer after the etching process forms a spacer against each side wall of the oxide line pattern. The width of the spacer can be measured. The etching is continued to under cut the deposition pattern and remove the tail ends of the spacers. The width of the spacer is measured again after the tail end is removed. From the width of the photoresist pattern, the width of the spacer before and after the tail end is removed, the width of the bottom under cut can be determined.